Date a Live, Vol. 4 (Light Novel): Sister Itsuka - Koushi Tachibana

nA spirit in flames, a town on fire... Shido was determined to save Kurumi and Mana. He simply had to. At the critical moment, though, he found himself immobilized, unable to keep his promise. Had Kotori Itsuka not shown up at the last second, everything would have been over... Nevertheless, Shido feels like his world has come crashing down, especially after discovering his little sister is a Spirit, too... Now he's been called to save her using the only method he knows: taking her on a date and making her fall for him. He needs to act fast--because he only has one day to complete this seemingly impossible mission!